Concrete remains one of the most versatile and popular construction materials in the world. Although it is rare for concrete structures to fail due to lack of intrinsic strength, gradual deterioration caused by lack of durability does occur and for this reason increasingly concrete structures are failing to last for their specified and expected lifetimes.
A major cause of concrete deterioration is the deleterious effects experienced by concrete as it undergoes cycles of freezing and thawing. Concrete is a porous material. When concrete is being made, as the cement and water in fresh concrete react to form a hardened paste material binding the coarse and the fine aggregates together, voids are left in the originally water-filled space between the cement grains. These voids are known as capillary pores with a size range from approximately 5 nm to 1 mm and sometimes even larger. In addition to these capillary pores, cement paste also contains are significant volume of smaller pores called gel pores (the term "gel" being used because some of the properties of the cement hydrates are similar to those of a gel). Water retained in such pores and the effects of capillary actions in such small pores have a major effect on the durability of hardened concrete, especially when concrete is subjected to repeated freeze/thaw cycles.
Concrete deterioration caused by freezing and thawing is of course linked to the presence of water retained in the pores, but cannot simply be explained by the expansion of water on freezing. While pure water in the open freezes at 0.degree. C., in concrete the water is really a solution of various salts that lower the freezing point. Moreover the temperature at which water in concrete can freeze varies as a function of the size of the pores, with the freezing point decreasing with the size of the pores--and since concrete has pores of various sizes there is no one single freezing point. Indeed the very smallest gel pores are too small to permit the formation of ice, and the greater part of the freezing takes place in the capillary pores. Larger voids in the concrete--resulting from incomplete compaction are usually air-filled and are not appreciably subjected to any freezing effects.
When water freezes there is an increase in volume of approximately 9%. As the temperature of concrete drops, freezing occurs gradually so that the unfrozen water in the capillary pores is subjected to hydraulic pressure caused by the volume expansion of ice. Such pressure, if not relieved, can result in internal stresses of sufficient magnitude to cause local failure of the concrete. This can occur, for example, in porous, saturated concrete containing no empty voids into which the liquid water can move. On subsequent thawing, the expansion caused by ice is maintained so that there is now new space available for additional water which may subsequently be imbibed. During re-freezing further expansion occurs and so repeated cycles of freezing and thawing have a cumulative effect. It is this repeated freezing and thawing that has a deleterious effect, rather than a single occurrence.
Two other processes contribute to the increase of hydraulic pressure of the unfrozen water in the capillaries. Firstly, since there is a thermodynamic imbalance between the gel water and the ice, diffusion of gel water into capillaries leads to a growth in the ice body and thus to an increase in hydraulic pressure. Secondly, the hydraulic pressure is also increased by the pressure of osmosis brought about by local increases in solute concentration due to the removal of frozen (pure) water from the original solution.
The extent of damage caused by repeated freeze/thaw cycles varies from surface scaling to complete disintegration as layers of ice are formed, starting at the exposed surface of the concrete and progressing through its depth. In general, concrete members which remain wet for long periods are more vulnerable.